1. Field of the Invention
The present invention relates to a method of preparing a volume type phase hologram film, and more particularly to a method of preparing a volume type phase hologram film having sufficient flexibility and further a superiority in high diffraction efficiency, high transparency and high moisture resistance, the method being capable of being used in various purposes.
2. Related Background Art
Holography is a unique technique of forming an optical image by which an object is irradiated by a well coherent wave such as a laser beem where the wave is modulated in amplitude and phase in accordance with the shape of the object, the modulated wave reflected upon or transmitted through the object is recorded (=hologram) and the hologram is irradiated again by the laser beam so as to reproduce an optical image of the original object.
With the recent development of study on holography, it has been made clear to a certain extent what material is suitable for use in holography or what characteristics such a hologram recording material should have.
Thus, there have already been proposed various materials such as bleached silver salt (U.S. Pat. No. 3,672,744), dichromate gelatine (U.S. Pat. No. 3,617,274), photoresist, thermoplastic resin, inorganic glass materials and ferroelectric substances, and at present a further study on the properties of these materials is proceeding in the art.
The properties which a hologram recording material (or sensitive material) should have may be summarized as follows:
(1) to have a sensitivity to a laser beam, especially to a laser beam in the visible wavelength region and to have a high sensitivity at the same time;
(2) to have a high resolving power;
(3) to give a hologram of high diffraction efficiency;
(4) to give a hologram of low noise level;
(5) to give a stable hologram; and
(6) to allow easy recording and reproducing operations.
As will be seen from the above, the requirements for a hologram recording material are very severe ones.
In view of practical purposes, very few known hologram recording materials can satisfy the above requirements completely or at least partially to the extent that its use may be practical.
Among the above mentioned materials, bleached silver salt and dichromate gelatine may be considered to be practically usable. However, they have particular disadvantages. The former necessitates a bleaching treatment in adition to ordinary treatments and furthermore the hologram obtained from it is poor in light fastness. The latter has a difficulty regarding the preservation of hologram because the hologram obtained from this material lacks adequate stability against moisture.
To overcome the disadvantages the conventional holograms have had, U.S. Pat. No. 4,287,277 discloses a hologram employing polyvinyl carbazole. This hologram employing polyvinyl carbazole is suitably used as it can satisfy the properties (1) to (6) mentioned above.
In general, volume type phase holograms are holograms in which a Bragg's grating is formed in the inside of a polymer and diffraction light wavelength is controlled by the grating distance (or grating space), where the grating pattern comprising several thousand or more lines is formed in a thickness of several ten .mu.m with a grating space of 0.1 to 0.2 .mu.m. Therefore, at the time of manufacturing a volume type phase holograms (particularly at the time of exposure to laser beam), the exposure to laser beam must be carried out in a stationary state so that fluctuation may be suppressed within approximately 0.01 to 0.02 .mu.m.
More specifically, at the time of the exposure to light, vibration, air-swirling caused by the passage of a laser beam, deformation of a holder of a sensitive material, thermal expansion, density gradient owing to temperature of air, etc. must be made sufficiently small. For the purpose a rigid and transparent material is used as a support for supporting the sensitive material, and in general usually used is glass.
However, the holograms for which glass is used as the support can be applied only to uses of very limited scope, such as displays or the like.